Holocene palynological sequences and palaeoenvironmental changes in the Bohai Sea area

Palynological analyses combined with AMS14C dating were carried out in a gravity core BH-264 retrieved from a mud depositional system in the Huanghe River underwater delta(the Bohai Sea) to investigate the paleoclimate changes during the Holocene.In the early Holocene(9.5—7.6 ka BP) the pollen and spore assemblages indicate a warm and humid climate.During this stage,the Bohai seashore area is mainly composed of marshes as the vegetations mainly consisted of Cyperaceae and Typhaceae,and the surrounding mountainous,hilly and flat areas were mainly covered by Pinus and Deciduous quercus accompanied with Betula,Ulmaceae and Moraceae.In the mid-Holocene(7.6—5.8 ka BP),due to the increase of temperature and sea-level,Pinus dominated in the pollen and spore assemblages.However the content of pollen genus such as herb,which is not favourable totransportation for long distances,decreases to the lowest level in the Holocene in this warm stage.The East Asian summer monsoon(EASM) has decreased since 5.8 ka BP and as a result the rainfall in the surrounding area of Bohai Sea reduces and an arid environment dominates.Such xeric genus as Chenopodiaceae,Dendranthema,Artemisia are dominant in the seashore environment.Moreover,the formation of the Huanghe(Yellow River) Delta shortened the distance between the pollen supplying area and the investigation area,which resulted in higher efficiency for the transportation of pollen and spore to the sediment core BH-264.